1. Field of the Invention
The present invention relates generally to an ultra wideband wireless communication system and, more particularly, to an ultra wideband M-ary Code Shift Keying/Binary Pulse Position Modulation wireless communication system and method, in which a Binary Pulse Position Modulation scheme combined with an M-ary Code Shift Keying scheme is applied to the modulation of ultra wideband signals, thus improving the information rate.
2. Description of the Related Art
Recently, the rapid development of wireless communication technology and the popularization of wireless devices have significantly influenced humans' lifestyles. In particular, research into Ultra Wideband (UWB) communication that can coexist with conventional wireless communication service without the need for separate frequency resources and perform high-speed wideband wireless communication has been actively carried out.
UWB communication based on impulse radio technology receives and transmits data using short pulses (Gaussian monocycle), and has a wide bandwidth of several GHz when viewed in the frequency domain, since very short pulses are used.
Such UWB communication is advantageous in that low power is consumed because it transmits and receives data without using a carrier, unlike conventional communication, and it can be used without affecting other devices because it uses an ultra wideband and, therefore, signals are detected at a level lower than the noise level in the frequency domain.
Meanwhile, UWB is also advantageous in that the duty cycle of pulses is very low, so that the transmission rate is very low, multiple access is possible and the influence of interference attributable to multipath can be suppressed.
Although UWB may be applied to various fields, current principal research focuses on high-speed local area communication in a range of from about several to about several dozens of meters. Since UWB communication can deal with high-speed data transmission and reception, ultrahigh quality images, such as digital high vision broadcasting or Digital Versatile Disk images, can be transmitted in a streaming fashion.
Such UWB technology is a promising solution to indoor short-range wireless communication and a UWB system can effectively deal with multipath, so that the UWB technology can support both a large amount of data and multiple access performance.
The reason for using the UWB system is that the UWB system has a structure superior to a conventional system due to low power per unit bandwidth or low power radiation and is implemented in a simple structure, so that it is easy to generate signals.
Time Hopping Pulse Position Modulation (TH-PPM), a currently used modulation scheme, can support the above-described requirements. However, research into interference occurring in conventional systems has been continuously carried out so as to further increase the data rate while maintaining the same multiple access performance and minimizing interference occurring in conventional narrowband systems.
The improvement of the above-described problem should not violate the spectral mask requirements specified in “First UWB Report and Order” issued by the Federal Communications Commission (FCC) or affect the implementation of the low complexity of a system.
The conventional TH-PPM system modulates 1-bit data using Binary Pulse Position Modulation (BPPM), and reduces catastrophic collisions attributable to multipath using a user-specific TH code.
There have been many attempts to increase the data rate by changing the system based on such a basic structure.
Two current main approaches focus on either the transmission of different pulse shapes or the modification of modulation format.
The first approach is referred to as Pulse Shape Modulation (PSM), and is based on the transmission of orthogonal modified Hermite Pulses using BPPM. In this system, each user is assigned a set of orthogonal pulses. The generation of such a set of orthogonal pulses is based on the differentiation of pulses.
Accordingly, when the size of the set increases, the zero-crossing rate of pulses increases. This requires additional hardware that generates pulses. Furthermore, zero-crossing is increased due to the generation of such pulses, so that the complexity of the hardware is increased, thus limiting the size of the orthogonal set and, therefore, limiting the data rate.
By using PSM, the size of the orthogonal set is increased and, therefore, the data rate (which does not affect multiple access performance) is increased, in which case each pulse of the set exhibits various spectral characteristics. This requires different antenna structures at a receiver and violates the spectral mask requirements.
Furthermore, an increase in data rate results in a degradation in Bit Error Rate (BER) performance, which can be overcome using channel coding. In this case, the transmission of redundant bits is required, so that the data rate can be reduced.
The second approach is based on the modification of the BPPM format to M-ary PPM (MPPM). If the pulse is shifted to M different locations, multiple access performance is reduced for a fixed bandwidth when the data rate increases.
When MPPM is employed, the same BER performance can be obtained for an increased data rate while the number of users is reduced.
Accordingly, the increased data rate based on the same BER performance is limited by the number of users (or multiple access performance) that should be accommodated in the system. Otherwise, transmission bandwidth must be increased at the expense of an increase in data rate.